1. Field of the Invention
The present invention relates to a sheet feeder on which a user can manually place a sheet and an image forming apparatus provided with such a sheet feeder.
2. Description of the Related Art
An image forming apparatus such as a printer includes, in many cases, a sheet feed path for automatically feeding a sheet from a sheet cassette mounted in an apparatus main body and a sheet feed path for feeding a sheet manually placed on a manual feed tray by a user. Usually, widely used A4 size sheets are fed from the sheet cassette and sheets of a variety of sizes such as a postcard size and an A3 size are fed from the manual feed tray. Usually, the manual feed tray is mounted to be openable and closable with respect to a housing of the apparatus main body, normally in a closed state and set to an opened state at the time of use.
A lift plate for bringing a manually placed sheet to a feed roller is attached to the manual feed tray. This lift plate is moved upward and downward by being lifted up to bring a sheet to the feed roller when the sheet is fed while being released from its lifted position and lowered when no sheet is fed. Such upward and downward movements are performed by a push-up cam arranged on the rear surface of a lift plate in a conventional sheet feeder (prior art apparatus 1). This push-up cam is driven by a driving system including a motor and a drive gear. Besides this, there is also known a sheet feeder (prior art apparatus 2) constructed such that a spring is arranged on the rear surface of a lift plate to constantly bias the lift plate in a direction toward a feed roller.
An unexpected force may act on the lift plate of the manual feed tray. For example, a user may press the manual feed tray with a large force after placing a bundle of sheets on the manual feed tray. In this case, in the prior art apparatus 1, the driving system may be damaged by an external force if the lift plate is in a state lifted up by a mechanism such as the push-up cam.
On the other hand, in the sheet feeder including the spring like the prior art apparatus 2, there is no problem of damage since the lift plate easily escapes if an external force as described above acts. However, in a sheet feeder dependent only on a spring load, it is difficult to give a sheet feed pressure corresponding to the thickness of the bundle of sheets to a feed roller and accuracy of spring loading needs to be increased. In addition, it is necessary to add a mechanism for preventing the lift plate from being inclined when a bundle of sheets is set on the manual feed tray. These requirements lead to a cost increase of the sheet feeder.